Abstract For patients with advanced endometrial cancer, the current responses are partial and of short duration; over 7,000 women will die of endometrial cancer this year. Hence, there is a critical need to test new drugs and develop more effective regimens. Targeted therapy, such as the use of small molecules that inhibit the molecular pathways leading to endometrial proliferation, provides a new opportunity to optimize treatment for endometrial cancer. However, such therapies require careful consideration as to which pathways are promoting proliferation in the tumor under therapy and what compensatory pathways may be activated in response to treatment. In the first grant period, we identified three major linked pathways to proliferation in the endometrium: These pathways interconnect and form a network composed of EGFR/cErbB2, mTOR/Akt, and VEGF/VEGFR and their downstream targets. We hypothesize and have data to support that blocking one receptor with a targeted molecule, such as gefitinib for EGFR, results in the selection of tumor cells that activate the other compensatory pathways in the network, such as through mTOR/Akt. In this grant period, we propose to map the pathways of resistance that are induced when each of the three pathways are blocked independently and together in the laboratory using in vitro cell and in vivo animal models and link these findings with ongoing clinical trials through the GOG. Specifically, in Aim 1 we will evaluate the molecular events relating to sensitivity and resistance in response to the blockade of the three critical endometrial growth pathways, EGFR/ErbB2, VEGF, and mTOR/Akt. For these studies, we will use the inhibitors lapatinib, bevacizumab, rapamycin and its derivative temsirolimus in well- established cell models for type I and II endometrial cancers. In Aim 2, we will correlate the markers of sensitivity and resistance (determined from Aim 1) for the three critical endometrial growth pathways, EGFR/ErbB2, VEGF, and mTOR/Akt, with the therapeutic effectiveness of the small molecule inhibitors in athymic mice xenografted with tumors from (a) the cell lines studied in Aim 1 and (b) patient-derived endometrial cancer specimens that comprise our Viable Tumor Tissue Bank. In Aim 3, we will determine whether identified markers for sensitivity and resistance from Aims 1 and 2 segregate with response in patients treated with lapatinib, bevacizumab, and temsirolimus as single agents. These studies will form the basis for the rational design of multi-agent targeted regimens in the future which have the potential to significantly enhance therapeutic options for women with advanced endometrial cancer. Project Narrative Endometrial cancer is the most frequent invasive gynecologic malignancy and the fourth most common cancer in women; for metastatic or recurrent disease, present therapies are not successful. Targeted therapy, such as the use of small molecules that inhibit the molecular pathways leading to endometrial proliferation, provides a new opportunity to optimize treatment for endometrial cancer. However, such therapies require careful consideration as to which pathways are promoting proliferation in the tumor under therapy and what compensatory pathways may be activated in response to treatment. The goal of this grant is to study inhibitors of the three central proliferation pathways in the endometrium and establish markers of sensitivity and resistance so that the appropriate patients can be chosen who will benefit from these new agents.